Inorganic carbonate composites as potential high temperature CO₂ sorbents with enhanced cycle stability

A calcium magnesium carbonate composite (CMC) material containing highly porous amorphous calcium carbonate (HPACC) and mesoporous magnesium carbonate (MMC) was synthesized. CMCs with varying HPACC : MMC mol ratios and high BET surface area (over 490 m² g^-1) were produced. The CMCs retained the morphology shared by HPACC and MMC. All these materials were built up of aggregated nanometer-sized particles. We tested the CO₂ uptake properties of the synthesized materials. The CMCs were calcined at 850 °C to obtain the corresponding calcium magnesium oxide composites (CMOs) that contained CaO : MgO at different mol ratios. CMO with CaO : MgO = 3 : 1 (CMO-3) showed comparable CO₂ uptake at 650 °C (0.586 g g^-1) to CaO sorbents obtained from pure HPACC (0.658 g g^-1) and the commercial CaCO₃ (0.562 g g^-1). Over 23 adsorption-desorption cycles CMOs also showed a lower CO₂ uptake capacity loss (35.7%) than CaO from HPACC (51.3%) and commercial CaCO₃ (79.7%). Al was introduced to CMO by the addition of Al(NO₃)₃ in the synthesis of CMC-3 to give ACMO after calcination. The presence of ∼19 mol% of Al(NO₃)₃ in ACMO-4 significantly enhanced its stability over 23 cycles (capacity loss of 5.2%) when compared with CMO-3 (calcined CMC-3) without adversely affecting the CO₂ uptake. After 100 cycles, ACMO-4 still had a CO₂ uptake of 0.219 g g^-1. Scanning electron microscope images clearly showed that the presence of Mg and Al in CMO hindered the sintering of CaCO₃ at high temperatures and therefore, enhanced the cycle stability of the CMO sorbents. We tested the CO₂ uptake properties of CMO and ACMO only under ideal laboratory testing environment, but our results indicated that these materials can be further optimized as good CO₂ sorbents for various applications.